Add UMA to track why a download is non-parallel when enabled parallel

content/browser/download/download_stats.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Holds helpers for gathering UMA stats about downloads.
 
 #ifndef CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_STATS_H_
 #define CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_STATS_H_
 
 #include <stddef.h>
@@ skipping 127 matching lines @@
 };
 
 enum DownloadDiscardReason {
   // The download is being discarded due to a user action.
   DOWNLOAD_DISCARD_DUE_TO_USER_ACTION,
 
   // The download is being discarded due to the browser being shut down.
   DOWNLOAD_DISCARD_DUE_TO_SHUTDOWN
 };
 
+// The reason why parallel download is not applicable and fallbacks to a
+// non-parallel download.
+enum ParallelDownloadFailReason {

[Ilya Sherman, 2017/04/07 22:13:08]

nit: Could this be an enum class?

[xingliu, 2017/04/08 01:13:28]

Done.

+  STRONG_VALIDATORS = 0,
+  ACCEPT_RANGE_HEADER,
+  CONTENT_LENGTH_HEADER,
+  FILE_SIZE,
+  CONNECTION_TYPE,
+  FAIL_COUNT,

[Ilya Sherman, 2017/04/07 22:13:08]

What does "FAIL_COUNT" mean?  "EXCEEDED_MAX_ALLOWED_FAILURES"?

[xingliu, 2017/04/08 01:13:28]

Changed to FALLBACK_TO_NORMAL_DOWNLOAD_COUNT.

+  MAX_COUNT,

[Ilya Sherman, 2017/04/07 22:13:08]

nit: I find "MAX_COUNT" to be a confusing name.  Could this just be "COUNT"? 
(Assuming the name above doesn't need "count" in it.)

[xingliu, 2017/04/08 01:13:28]

Done. Reworked the naming and the comments.

+};
+
 // Increment one of the above counts.
 void RecordDownloadCount(DownloadCountTypes type);
 
 // Record initiation of a download from a specific source.
 void RecordDownloadSource(DownloadSource source);
 
 // Record COMPLETED_COUNT and how long the download took.
 void RecordDownloadCompleted(const base::TimeTicks& start,
                              int64_t download_len);
 
@@ skipping 71 matching lines @@
 void RecordParallelDownloadAddStreamSuccess(bool success);
 
 // Records the bandwidth for parallel download and estimates the saved time at
 // the file end. Does not count in any hash computation or file open/close time.
 void RecordParallelDownloadStats(
     size_t bytes_downloaded_with_parallel_streams,
     base::TimeDelta time_with_parallel_streams,
     size_t bytes_downloaded_without_parallel_streams,
     base::TimeDelta time_without_parallel_streams);
 
+// Records the reason why the download is not created as parallel download.
+void RecordParallelDownloadFailReason(ParallelDownloadFailReason reason);

[qinmin, 2017/04/07 21:00:52]

This sounds more like a download failed. what about
RecordParallelRequestCreationFailureReason or sth like that

[xingliu, 2017/04/08 01:13:27]

Done.

+
 // Record the result of a download file rename.
 void RecordDownloadFileRenameResultAfterRetry(
     base::TimeDelta time_since_first_failure,
     DownloadInterruptReason interrupt_reason);
 
 enum SavePackageEvent {
   // The user has started to save a page as a package.
   SAVE_PACKAGE_STARTED,
 
   // The save package operation was cancelled.
@@ skipping 28 matching lines @@
 
 void RecordDownloadConnectionSecurity(const GURL& download_url,
                                       const std::vector<GURL>& url_chain);
 
 void RecordDownloadSourcePageTransitionType(
     const base::Optional<ui::PageTransition>& transition);
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_STATS_H_